Volume 21, Issue 8, 01 August 1953
Index of content:
21(1953); http://dx.doi.org/10.1063/1.1699212View Description Hide Description
The lifetimes of fluorescent substances are determined by exciting the fluorescence with light modulated at a high frequency and measuring the difference in phase between the exciting light and the resulting fluorescence. The exciting light is modulated by an ultrasonic standing wave in a liquid which causes the latter to act as an intermittent diffraction grating.Photomultiplier tubes are used to pick up the exciting and fluorescent light. Tuned circuits are used to isolate a single harmonic of the signal from the phototubes and the phase shift determined by comparison with a calibrated phase shifting circuit. The phase angle φ is related to the lifetime by tanφ=ωτ. In which ω is 2π times the frequency of modulation and τ is the lifetime of the excited state. The data show that lifetimes can be determined with a precision of 1 to 2×10−10 second.
The lifetimes of acridone in neutral solution (saturated to 10 percent saturated) and quinine sulfate (4×10−6 to 4×10−5 M) in 0.01 M nitric acid are independent of the concentration. The values found are acridone 1.59±0.01×10−8 second, and quinine sulfate 2.28×0.02×10−8 second. The lifetime of fluorescein in 0.005Mpotassium hydroxide increases with concentration over the range 10−6 to 5×10−4 M. This fact can be explained by assuming that some of the fluorescent light is absorbed and re‐emitted. The lifetime of fluorescein extrapolated to zero concentration is 4.5±0.1×10−9 second.
21(1953); http://dx.doi.org/10.1063/1.1699213View Description Hide Description
The Debye‐Hückel equations are solved for the case of a polarized electrode, yielding, for the mean ionic concentration at a distance x from a plane cathode immersed into an aqueous solution of a uni‐univalent electrolyte of concentration aN 0 molecules/cm3,1/K being the Debye length. The mean local charge density and the mean local potential are calculated, and it is shown that the latter quantity is merely the thermodynamical potential resulting from the increased activity of the cations near the electrode, the system behaving as a concentration cell.
21(1953); http://dx.doi.org/10.1063/1.1699214View Description Hide Description
In Debye's theory of polar fluids, the electrostaticinteraction of the molecules was taken into account by introducing the Lorentz local field formula. This formula is inaccurate, especially for liquids. More recent treatments are of two types. One, developed by Onsager, treats one molecule explicitly and replaces the others with a continuum. The other, developed by Kirkwood, uses the statistical mechanical theory of strongly interacting molecules. The second method is rigorous and can be used to derive the approximate results of the first; nevertheless, the first has remained popular because of its simplicity. The treatment presented here attempts to combine the rigor of Kirkwood's method with the simplicity of Onsager's. This is done by use of several useful theorems of classical electrostatics, which are overlooked when dielectrics are treated in the traditional textbook manner. The result is shown to be equivalent to Kirkwood's; but it is in a more flexible form, and it can, therefore, be used to derive approximations of a more general class.
Effect of Isotopic Substitution on the Entropy, Enthalpy, and Heat Capacity of Ideal Gases. I. Systems in Thermal Equilibrium. II. Chemically Reacting Systems21(1953); http://dx.doi.org/10.1063/1.1699215View Description Hide Description
The formulas of Bigeleisen and Mayer for the free energy difference between isotopic compounds are used to derive formulas for the similar differences in entropy,enthalpy, and heat capacity. Tables of these functions are calculated. The analogous quantities in the ``thermodynamics of reaction rates'' are discussed. Several useful formulas for the entropy differences of isotopic molecules are derived.
21(1953); http://dx.doi.org/10.1063/1.1699216View Description Hide Description
The pure rotation spectra of HCl, DCl, HBr, and NH3 in the region from 40 to 140 microns as measured with an automatic‐recording grating spectrometer are presented in tabular and graphical form. A photograph of a recorded spectrogram provides an indication of the quality of the data. A comparison of the experimental results with figures based on accepted values of the molecular constants shows substantial agreement.
21(1953); http://dx.doi.org/10.1063/1.1699217View Description Hide Description
The Onsager‐Fuoss equation for the concentration dependence of the diffusion coefficient is derived from a kinetic picture, taking into account the asymmetry in the Brownian motion induced by concentration gradients. Mass flow is introduced by considering diffusion in the presence of a pressure gradient. Systems exhibiting volume changes on mixing are discussed, and a modification of the Matano procedure for calculating diffusion coefficients is suggested. It is shown that diffusion coefficients obtained from steady‐state permeability measurements and those obtained from interdiffusion data differ by a factor of V̄ 2 c 2, the volume fraction of the nonpermeating component.
21(1953); http://dx.doi.org/10.1063/1.1699218View Description Hide Description
Just as is the case with ordinary diffusion,thermal diffusion is accompanied by a mass flow, which does not vanish even after the steady state has been reached. As a result of this mass motion the thermal diffusion coefficients of both components in a binary mixture enter into the expression for the steady‐state mole fraction gradient of either one, so that a knowledge of the latter is not sufficient, in general, to resolve the total heat of activation for diffusion into a heat of hole formation and a heat of activation for jumping. Mass flow should exist even in pure liquids in the presence of a temperature gradient, and this gives rise to the interesting possibility that the heat of activation for self‐diffusion can be resolved into its components.
21(1953); http://dx.doi.org/10.1063/1.1699219View Description Hide Description
A theory which relates the dielectric polarization of polar gases to the intermolecular potential is formulated. In it occurs an expansion in powers of 1/V, analogous to the virial expansion. The convergence of this series is briefly discussed, and the impossibility of rotational transitions in the gas phase is shown. Evaluation is limited to the leading term of the expansion, which involves interactions of a pair of molecules only and which is calculable from a knowledge of the intermolecular potential. Various intermolecular potentials consistent with virial data are introduced, and the results are compared with experiment for ammonia, methyl fluoride, and hydrogen fluoride. In view of the unsatisfactory precision of the experimental data, the agreement is as good as can be expected and is qualitatively in agreement with previous concepts of the strength and nature of the hydrogen bond.
21(1953); http://dx.doi.org/10.1063/1.1699220View Description Hide Description
Diffusion coefficients have been measured in the system CH4—TCH3 to 300 atmospheres pressure. In general the results are consistent with the predictions of Enskog's dense gas theory. At the highest densities, the diffusion coefficients are lower than the theory predicts. This is expected as, at these densities, unevaluated terms in the dense gas correction become significant.
21(1953); http://dx.doi.org/10.1063/1.1699221View Description Hide Description
Diffusion coefficients have been measured for the system CCl4–SnI4 using I131 tagged SnI4. Isotherms were obtained at 50°C and 75°C to 2000 atmospheres pressure. The results are interpreted in terms of the activation enthalpy,entropy, and volume. Below 1000 atmospheres the diffusion mechanism differs only slightly from that in effect at atmospheric pressure. At higher pressures orientational effects are observed.
21(1953); http://dx.doi.org/10.1063/1.1699222View Description Hide Description
Diffusion coefficients have been measured in liquid sulfur over the range 120°C—320°C. Two varieties of sulfur, both treated to remove organic impurities, were used. No essential difference was found between them. The break in the diffusion rate occurs at a lower temperature than does the break in the viscosity curve, and the total drop in diffusion rate is much smaller than the rise in viscosity. The effect of added impurity is also less important for diffusion than for flow. This is attributed to the smaller effective moving segment in molecular motion.
21(1953); http://dx.doi.org/10.1063/1.1699223View Description Hide Description
Diffusion coefficients have been measured in liquid sulfur to 2500 atmospheres pressure for isotherms at 140°C, 150°C, 175°C, and 210°C. One isotherm was also obtained at 195°C for sulfur containing 0.75 percent iodine. The motion in the viscous region is characterized by very small activation volumes which indicates that the moving segment is very small. This shows why the drop in diffusion rate in the transition region is small compared with the increase in viscosity. The iodine reduces the ``effective temperature'' 10–15°C. Above 1200–1500 atmospheres only one form of liquid sulfur seems to exist.
21(1953); http://dx.doi.org/10.1063/1.1699224View Description Hide Description
The number of clusters of a given size in a real gas frequently turns out to be negative. Nevertheless a model based upon the cluster idea leads to the correct virial expansion. This model is tested further by a direct calculation of the gas imperfection entropy from the possibly complex entropy of formation and of mixing of these clusters.
21(1953); http://dx.doi.org/10.1063/1.1699225View Description Hide Description
The fine structure of some parallel bands of the symmetric top molecules CH3I, CD3Br, CHF3, and CHCl3 has been investigated in the 1.6 and 2.2μ regions. The double‐pass plane grating spectrograph was capable of achieving a resolving power in excess of 100 000. For most of these bands the ``line'' nature of the sub‐bands in the P, Q, and R branches was demonstrated. In the case of CH3I the resolution was complete enough to demonstrate conclusively the complicated fine structure of the P and R ``lines.'' Molecular constants have been derived from the bands of CH3I, CD3Br, and CHF3. The theoretically expected relationship between the zeta's has been shown to hold in perpendicular CH3I bands.
21(1953); http://dx.doi.org/10.1063/1.1699226View Description Hide Description
A mass spectrometric method employing a molecular beam gas sampling system has been developed for the detection of atoms and radicals in chemical reactions. Background signals have been virtually eliminated by mechanically modulating the molecular beam and applying phase detection to the ion signal. The application of the method is illustrated by examples of low‐pressure flames. In the hydrogen‐oxygen flame H, O, and OH have been positively identified. The mass spectrum of a simple hydrocarbon flame, such as the methane‐oxygen flame, is complicated by the presence of a large number of stable components generated in the flame. In the methane‐oxygen flame the stable intermediates include C2H2, CO, CH2O or C2H6, CH4O, and C4H2. The methyl radical has been clearly identified in the methane‐oxygen flame. A search for the HO2 radical in the hydrogen‐oxygen flame was made without obtaining positive results. The HO2 detection problem is discussed in detail.
21(1953); http://dx.doi.org/10.1063/1.1699227View Description Hide Description
The average coefficients of thermal expansion of hexagonal zirconium between 298°K and 1143°K are 5.5×10−6 deg−1 along the a axis, 10.8×10−6 deg−1 along the c axis, and 7.2×10−6 deg−1 for a randomly oriented polycrystalline sample. The average value of the linear coefficient of expansion of cubic zirconium between 1143°K and 1600°K is 9.7×10−6 deg−1. At the transition there is a decrease in volume of 0.66 percent.
21(1953); http://dx.doi.org/10.1063/1.1699228View Description Hide Description
The equilibrium curve for two of the phases of CaCO3, calcite‐aragonite, has been determined as a function of temperature and pressure over the range 25–80°C and several thousand kg/cm2, using the electrical conductivity of aqueous solutions of each form. Aragonite is proved to be the high pressure modification. From the determined equilibrium curve, various thermodynamic quantities are obtained in a more accurate fashion than previously possible. All normal natural occurrences of aragonite must be due to metastable formation rather than equilibrium, since the geothermal gradient lies completely in the calcite field.
21(1953); http://dx.doi.org/10.1063/1.1699229View Description Hide Description
The accuracy of the quasi‐chemical approximation used in the theoretical treatment of a lattice model for solutions in which cooperative orientation of molecules takes place is examined. The quasi‐chemical method of Guggenheim and the modified quasi‐chemical method of Kikuchi are used to find higher approximations. It is found that the quasi‐chemical approximation for pairs is satisfactory for lattices which are not closepacked, though it may be seriously in error for closepacked lattices.
21(1953); http://dx.doi.org/10.1063/1.1699230View Description Hide Description
A theoretical approach to the problem of the interconnection between adsorption on proteins and interactions between protein molecules in solution is outlined. As an illustration, the second virial coefficient and related quantities are discussed. A more detailed account of this work will be published in the Annals of the New York Academy of Sciences.